The present invention relates prostheses for human joints, and more particularly to prostheses for knee, elbow, or other joints of the body, and more particularly to a method for designing the same. In particular, the present invention has a preferred application as a knee prosthesis. The movement of the knee joint in flexion and extension does not take place in a simple hinge-like manner, as in some other joints, but in a complicated movement, that includes displacements and rotations, so that the same part of one articular surface is not always applied to the same part of the other articular surface, and the axis of motion is not fixed. Furthermore, the knee joint is formed between the longest bones of the body, consequently with high lever arms relative to the foot and to the hip, and therefore, the forces and moments across the joint at the interface between the articulating surfaces exceeds that of any other joint in the body.
Attempts to implant knee prostheses employing a metal hinge and intramedullary stems for anchoring the hinge to the femur and tibia date back to the 1930's. Because of the complexity of the knee joint action, and the forces and moments across the joint, these prostheses were unsuccessful. The elements were subject to wear resulting in dispersion of metal into the surrounding tissue with consequent complications, and in high stresses at the implant-bone interfaces resulting in bone resorption, pain, and implant failure. However, a hinge does not permit ready access of natural lubrication to the joint, and, by its nature, permits rotation only through a single plane. It cannot duplicate the complex movements of the knee joint. Thus, a less than satisfactory result is inevitable. See, e.g., D. V. Girzadas et al., "Performance of a Hinged Metal Knee Prosthesis", J. Bone and Joint Surgery, Vol. 50-A, No. 2, March 1968, pp. 355 et seq.
Since that time, unlinked condylar replacement knee replacements have been designed. These allowed some freedom of motion between the femoral and tibial replacement surfaces. The first example was the polycentric knee by Gunston. (Gunston, J. Bone & Joint Surgery)
Another early design was by Ewald, in which he proposed surfaces representing the anatomical to allow normal joint motion. The Ewald prosthesis was an advancement over the earlier knees in that it permits rotation of the tibia with respect to the femur (i.e., pivoting of the medial condyle about the lateral condyle), and translation of the femur with respect to the tibia-movements in different planes at once (sagittal and transverse). All of those movements are necessary to duplicate the movement of a natural knee. For a detailed description of the control mechanism and the guiding components of the knee joint during normal extension and more particularly flexion, see O. C. Brantigan et al., "The Mechanics of the Ligaments and Menisci of the Knee Joint", J. Bone and Joint Surgery, Vol. XXIII, No. 1, January 1941, pp. 44 et seq.; and A. J. Helfet, "Control and Guide Mechanism of the Knee Joint", A.A.O.S. Instructional Course Lectures (1970), pp. 64-65.
However, although the Ewald prosthesis provided a more natural movement than the prior art knee joints, the Ewald knee was not designed to allow for the natural knee movement known as laxity. Laxity can be defined as the partially restrained motion or free play in a specified direction before substantial ligamentous restraint takes place at the extremes of motion (see generally, Markolf, K. L., et al, Journal of Bone & Joint Surgery, 63-A; 570-585, 1981; Walker, P. S. Ch. 4, p. 202-204, Human Joints & Their Artificial Replacements, pub. C. C. Thomas, Springfield, Ill., 1977). Since laxity is limited by certain ligaments that are resected during implantation of a prosthesis, primarily the cruciate ligaments, prior to the present invention, there was a need in the art for a prosthesis capable of duplicating the laxity characteristics of a natural knee joint.
Several designs allowed freedom of motion, by providing partial conformity between the femoral and tibial surfaces, using geometrical radii of curvature. These designs included the DuoPatella, Townley, Total Condylar, and Anametric. However, the shape of the surfaces is not related to the actual motion and laxity of the normal knee joint.
The femoral condyles of a knee prosthesis can be joined to the femur in several different ways. One method is to provide two pegs which insert into the trabecular bone of the medial and lateral femoral condyles. Another method is to provide a stem that projects from the center of the condyles and is inserted into and locked within the medullary canal of the femur. Because of the valgus angle of the femur, it was necessary to provide both right and left femoral components, which were not interchangeable with each other. In addition, certain situations require a short stem, while others require a long stem. Thus, it was necessary to maintain in stock four different femoral components--long stem and short stem versions of both left and right leg components.